In the ever-expanding universe of blogs, websites and Twitter feeds, it doesn't take much for a rumour to get going. Especially if, like Vin Diesel, you have more than 48 million Facebook followers. The story that Diesel is set to play Groot in Marvel Studios' forthcoming comic book space opera Guardians of the Galaxy was given life back in August, when the Fast and Furious star changed his Facebook photo to a shot of the talking tree-like alien.

First, Groot only ever says one thing – the immortal line: "I am Groot!" – in the comics. Is Diesel, one of Hollywood's best-known action stars, really planning to take a role that requires so little effort, and offers such meagre potential returns? Second, Marvel regularly makes a big song and dance about high-profile castings (see Bradley Cooper as Rocket Raccoon and Zoe Saldana as Gamora.) There's not been so much as a sniff of a press release from the Disney-owned studio about Diesel as Groot. Finally, the Guardians of the Galaxy shoot wrapped in London last week, and Diesel was not on set. The idea that he's going to do his entire mo-cap performance solo after the fact seems pretty far out.

I was lucky enough to watch a Guardians scene being filmed at Longcross studio in London last month. Though under strict instructions not to breathe a word about what I saw for the time being, it's only fair to mention that Diesel's supposed spot was being held for him by a substitute tree alien. And yes, all he said was: "I am Groot!" The film looks a bonkers-brilliant proposition, with some gorgeously-realised space environments and a brio to the dialogue and action that recalls the best of Marvel's work. Director James Gunn seems to be following the Joss Whedon rule, which says that no matter how preposterous your heroes are, audiences can be encouraged to identify with them if they feel like real (preferably funny and quick-witted) people.

Perhaps Diesel is just having a laugh. Or maybe – just maybe – Marvel themselves are engaged in a policy of deliberate misinformation to undermine the bloggers who have been giving them such a hard time recently. It's well documented that the studio got very upset indeed with influential site Latino Review when it broke the news that Guardians of the Galaxy was to be Marvel's next movie adaptation of a comic book. There were even rumblings of a legal intervention. Perhaps sending everyone up the leafy garden path and round the cosmic houses ultimately seemed a lot more fun?